The installer for Illustrator 10 will not work on a machine that does not have Classic (OS 9) support. You did not specify what kind of Mac you have so this is a simplified list: Illustrator CS(11)-CS2(12) will install and work properly with a PPC Mac (pre Intel.) Illustrator CS3(13)-CS4(14) will work properly on an Intel Mac.

Illustrator CS-CS2 has glitches when running via Rosetta on an Intel Mac and CS3-CS4 run natively on an Intel Mac.

There is not a free upgrade to any of these. Different number versions are always paid upgrades.

As long as you are not violating licensing issues *you being the one that the software is licensed to,* Illustrator 10 can work on an Intel Mac via Rosetta if it has been previously installed on a machine that has Classic support. You just copy the installed application folder from one machine to the other. You also have to copy the preference files because some of the user preferences cannot be changed. I run Illustrator 10 on my Mac Pros with Snow Leopard (10.6) and it seems to work well even with the caveats about preferences.

Post back with more details about your machine and other pertinent info if you want more detailed answers.

Glad I could be of some help. CS4 is definitely better than 10 on an Intel machine. Unfortunately you would not qualify for the upgrade price. The full retail price is $599. (Upgrade is $199.) You can search around and find various pricing. Just remember that you would need the standalone retail package and not the upgrade package. You have to have a CS-CS3 serial number to use the upgrade package.